1. Field of the Invention
This invention relates to positive photosensitive compositions for application of the lift-off technique and a method of forming patterns using the compositions. More particularly, the invention relates to positive photosensitive compositions for application to the lift-off technique with which half-a-micron order or even finer patterns such as conducting patterns and electrodes made of metals or metal oxides, as well as insulation patterns can be formed precisely on the substrate. The invention also relates to a method of forming patterns using such compositions.
2. Description of Related Art
Conducting patterns and electrodes made of various metals such as aluminum, copper, tantalum, nickel-iron (Ni--Fe) or metal oxides such as aluminum oxide, as well as insulation patterns are formed on semiconductor and other substrates by various known methods including the lift-off technique. A typical process of the lift-off technique proceeds as follows: a photosensitive composition is coated on a substrate to form a resist layer, which is exposed via a mask and developed to form a resist pattern on the substrate; thereafter, a metal or metal oxide layer is formed by blanket deposition such as sputtering or chemical vapor deposition onto the entire surface of the substrate including the resist pattern; removing away the resist pattern then "lifts off" the overlying metal or metal oxide layer but leaves the metal or metal oxide intact on the substrate as a conducting pattern or electrodes.
Without involving an etching step, the lift-off technique has the advantage that patterns can easily be formed of difficult-to-etch metals and metal oxides; on the other hand, the lift-off technique uses the resist pattern as a mask in forming conducting patterns and electrodes of metals and metal oxides and, hence, the precision of their geometry depends on the profile of resist pattern elements. Under the circumstances, a need has arisen for photosensitive compositions that are suitable for application of the lift-off technique and which are capable of precise formation of fineline conducting patterns, electrodes and insulation patterns. A particularly great need has arisen in recent years for photosensitive compositions to which the lift-off technique is applicable and which are suitable for use in the formation of conducting metallization patterns in semiconductor devices, as well as in the formation of metal or metal oxide electrodes (terminals) on magnetic heads in magnetic disk devices such as a giant magnetoresistive (GMR) head and a magnetoresistive (MR) head.
It is generally held that resist pattern desired for application of the lift-off technique should have such a feature profile that it has an undercut (i.e., the narrowing of the bottom of a pattern element at the interface with the substrate). See Unexamined Published Japanese Patent Application No. 69111/1996 and Ritsuko Nakano et al., Japanese Journal of Applied Physics, vol. 30, No. 11B, November 1991, pp. 3121-3124. The undercut is commonly referred to as a "micro-groov" and believed to occur for the following reason: if a positive photosensitive composition is used, only that area which has been irradiated with light becomes soluble in a liquid alkali developer and the resulting resist pattern should inherently have a rectangular feature profile; however, in practice, due to some reason, the bottom of the pattern element which should theoretically be shadowed dissolves in the liquid developer to have an "undercut".
The micro-groove is preferred in the application of the lift-off technique for several reasons, among which the following are worth mention: when a metal or metal oxide layer (metallization) forms not only onto the entire surface of the substrate including the resist pattern but, the metallization will readily get into the space between adjacent resist elements to thereby form a conducting pattern or electrodes having a rectangular or tapered cross-sectional profile that assures good adhesion to the substrate; the resist pattern can easily be stripped from the substrate; hence, no adverse effects will be exerted on the geometry of the conducting pattern and electrodes when the resist pattern is removed away. These advantages are particularly noticeable when the metal or metal oxide layer is to be formed by sputtering.
Although the micro-groove has these advantages, the cause of its generation has not been fully unraveled and it is difficult to ensure that a pattern profile having micro-grooves can be reproduced in high precision; in addition, the degree of undercut in the bottom of a pattern element due to the micro-groove has been difficult to control in accordance with a specific case.
Furthermore, the heretofore proposed photosensitive compositions capable of forming micro-grooves produce resist patterns with a narrowed top and, hence, are unsatisfactory in geometry. The resist patterns are also poor in resolution. In other words, the conventional photosensitive compositions which are capable of forming micro-grooves are suit- able for forming rough patterns of a feature size of about 10 .mu.m but not satisfactory for the purpose of precisely forming very fine (.ltoreq.2 .mu.m) patterns, particularly superfine patterns of half-a-micron order or even finer patterns.